1. Field of the Invention
The present invention relates to a paper transporting felt and a press apparatus of a paper machine having the paper transporting felt.
2. Description of the Related Art
Generally, a paper machine includes a wire part, a press part, and a drier part. The wire part, the press part, and the drier part are arranged in that order in a wet paper transporting direction. A wet paper is transported sequentially through wet paper transporting members provided in the wire part, the press part, and the drier part, water is squeezed out of the wet paper in the meantime, and the wet paper is finally dried on a drier canvas (as a paper transporting member) in the drier part. A press apparatus disposed in the press part includes a plurality of press mechanisms arranged in series in the wet paper transporting direction.
Each press mechanism includes a pair of paper transporting felts having an endless belt shape and a pair of rolls (that is, roll press) or a roll and shoe (that is, shoe press) as a press vertically opposed to each other to interpose a part of the pair of paper transporting felts therebetween. By allowing the pair of rolls or the roll and shoe to press the wet paper, which is transported by the paper transporting felts traveling in the same direction with the same speed, along with the paper transporting felt, water is squeezed out of the wet paper and is absorbed by the paper transporting felt. The paper machine is classified into a roll press type paper machine in which a press apparatus interposing and pressing a part of the paper transporting felts, which interpose the wet paper, between a roll and a roll is provided in the press part and a shoe press type paper machine in which a press apparatus interposing and pressing a part of the paper transporting felts, which interpose the wet paper, between a roll and a shoe is provided in the press part. Specifically, since the shoe press type paper machine can have a greater press zone of a pressing portion (that is, a nip) of the press than that of the roll press type paper machine which is used more widely, the pressing time can be more elongated. Accordingly, the shoe press type paper machine can have ma ore excellent dewatering characteristic.
In the paper machine, the wet paper transported by the paper transporting felts of the press apparatus is sequentially transported through the paper transporting felts of a plurality of press mechanisms arranged in series while squeezing out water therefrom and then the surfaces of the wet paper are smoothed. Accordingly, the paper transporting felts should have a function of transporting the wet paper, a function of squeezing water out of the wet paper, a paper separating function of smoothly separating the wet paper from the paper transporting felt for sending the wet paper to a next process (in other words, facilitating the peeling of the wet paper from the paper transporting felts), and a wet paper surface smoothing function of smoothing the surfaces of the wet paper. Specifically, since much water may be squeezed out of the wet paper by the press mechanism disposed on the upstream side in the wet paper transporting direction of the press part, the paper transporting felts of the press mechanism disposed on the downstream side in the wet paper transporting direction require the wet paper surface smoothing function rather than the function of squeezing water out of the wet paper as a more important function.
In a portion covering the center of the pressing portion of the press mechanism to the exit thereof, since a pressure applied to the wet paper and the paper transporting felts is abruptly released, the volumes of the paper transporting felt and the wet paper in the portion are abruptly expanded. As a result, a minus pressure is generated in the paper transporting felts and the wet paper and a capillary phenomenon acts thereon due to micro fibers constituting the wet paper, thereby resulting in a re-wetting phenomenon that the water absorbed by the paper transporting felts is transferred again to the wet paper. In this way, the portion covering the center of the pressing portion to the exit thereof serves as an important factor for deteriorating the dewatering performance of the press apparatus of the paper machine.
As a paper transporting felt for preventing the re-wetting phenomenon of the paper transporting felt and a blowing phenomenon at the time of press, there is known a paper transporting felt which is impregnated with emulsion resin and of which a wet paper side portion has been intensively studied (see U.S. Pat. No. 4,500,588.). In the paper transporting felt, more specifically, a barrier layer is formed by impregnating a batt layer formed on the surface of a base layer with the emulsion resin and performing a calendar machining to the wet paper side surface of the batt layer into a dense and smooth surface of a chamois leather type, or a rough fiber layer formed on the surface of the base layer is impregnated with the emulsion resin to form a barrier layer (non-woven fabric layer) on the rough fiber layer, and a fine fiber layer is formed on the barrier layer (non-woven fabric layer). Accordingly, since the barrier layer prevents the emulsion resin from reaching the wet paper side surface of the paper transporting felt, it is possible to prevent the re-wetting and the blowing of the paper transporting felt and thus to enhance a papermaking speed. In addition, when the paper transporting felt is pressed along with the wet paper in the pressing portion of the press mechanism, it is possible to reduce a factor of hindering the surface smoothing of the wet paper that air contained in the paper transporting felt is pushed out from the wet paper side surface to roughen the surface of the wet paper.
There is also known a paper transporting felt of which the surface is provided with a liquid impermeable layer (see German Utility Model Application Publication No. DE 29706427 U1.). The paper transporting felt has a fiber layer including thermoplastic fibers or melted fibers, and the thermoplastic fibers or the melted fibers are melted by heating the surface of the fiber layer to form the liquid impermeable layer.
U.S. Pat. No. 4,500,588 and German Utility Model Application Publication No. DE 29706427 U1 are referred to as related art.
The paper transporting felts fitted to a plurality of press mechanisms arranged in series have different main functions. The function of squeezing water out of the wet paper is mainly required for the paper transporting felt fitted to the press mechanism disposed on the upstream side in the wet paper transporting direction among the plurality of press mechanisms and the function of smoothing the surface of the wet paper is mainly required for the paper transporting felt fitted to the press mechanism disposed on the downstream side in the wet paper transporting direction. That is, the surface smoothness of the wet paper may mainly depend upon the paper transporting felt fitted to the press mechanism disposed on the downstream side in the wet paper transporting direction among the plurality of press mechanisms. Accordingly, the paper transporting felt fitted to the press mechanism disposed on the downstream side in the wet paper transporting direction among the plurality of press mechanisms should have a smooth wet paper side surface. However, since the wet paper input to the press mechanism disposed on the downstream side in the wet paper transporting direction among the plurality of press mechanisms slightly contains water, it is preferable that the paper transporting felt fitted to the press mechanism disposed on the downstream side should have a dewatering characteristic more or less.
The water contained in the wet paper is absorbed and squeezed out by the paper transporting felt which is compressed and decompressed in the pressing portion of the press mechanism. That is, the paper transporting felts require proper ventilation ability, compression ability, and decompression ability for efficiently squeezing out water.
In a press apparatus of the paper machine, specifically, in a press apparatus of a shoe press type paper machine or a single nip-type paper machine, since a pressing portion (that is, nip) of a press applies a very large pressure to the paper transporting felt to squeeze out water, the batt layer of the paper transporting felt coming into direct contact with the pressing portion of the press can be easily damaged and thus a lifetime of the paper transporting felt (that is, a usable period of time of the paper transporting felt) is short. Accordingly, maintenance for regularly interchanging the paper transporting felt with a new one is required.
In order to enhance the surface smoothness of a wet paper, it is difficult to merely replace the fiber constituting the surface of the batt layer of the paper transporting felt coming into direct contact with the wet paper with a micro fiber. Specifically, when the micro fiber is cut in a process prior to a needling process (a cutting operation is performed prior to the needling process and a fiber webbing sheet after the cutting operation is raised through needling) as one of processes of manufacturing the paper transporting felt, fiber lumps (that is, lumps of fibers) can be easily generated. Accordingly, the fiber lumps are raised from the belt through the needling right after the cutting operation and thus relatively large unevenness is formed on the surface of the paper transporting felt, thereby deteriorating the surface smoothness of the wet paper.
The paper transporting felt of which the wet paper side surface is processed, which is disclosed in U.S. Pat. No. 4,500,588, is a paper transporting felt having an excellent surface smoothness and a water impermeability (that is, ability of allowing water not to pass from the wet paper side surface to the press side surface), but has a poor ability of squeezing water out of a wet paper. Therefore, since a high ability of squeezing water out of the wet paper cannot be expected for the paper transporting felt, it is preferable so as to utilize the paper transporting felt that the dewatering of the wet paper is completely performed by a press mechanism disposed on the upstream side in the wet paper transporting direction. In other words, the paper transporting felt of which the wet paper side surface is subjected to the calendar process, which is disclosed in U.S. Pat. No. 4,500,588, can be preferably fitted to a press apparatus disposed on the downstream side in the wet paper transporting direction among a plurality of press mechanisms arranged in series. However, since much thermal energy is required for a heating and drying process in the drier part as a next process, it is important that the water in the wet paper is removed in the press part as much as possible. Therefore, it is more preferable that the paper transporting felt fitted to the press mechanism disposed on the downstream side in the wet paper transporting direction among a plurality of press mechanisms has a function of squeezing water out of the wet paper more or less. The paper transporting felt, which is disclosed in U.S. Pat. No. 4,500,588, in which the non-woven fabric layer is interposed between the rough fiber layer facing the base layer and the micro fiber layer facing the wet paper and having the fiber lumps is not suitable for the press apparatus of the paper machine in which the high ability of squeezing water out of the wet paper is required for the paper transporting felt.
In the paper transporting felt disclosed in German Utility Model Application Publication No. DE 29706427 U1, a liquid impermeable layer is formed by heating the surface of a fiber layer including thermoplastic fibers or melted fibers to melt the thermoplastic fibers or the melted fibers. The paper transporting felt has a feature that the thickness of the liquid impermeable layer can be controlled to some extent by adjusting the amount of heat applied from the surface of the fiber layer. However, since it is necessary to perform the heating within a range not deteriorating the fiber characteristic of the heating surface, that is, the surface of the liquid impermeable layer, by the heating, the depth of the paper transporting felt (that is, the thickness of the liquid impermeable layer) is limited in consideration of the thermal deterioration or thermal decomposition of the thermoplastic fibers or the melted fibers. Accordingly, the thickness of the liquid impermeable layer is automatically limited not to be too thick. In addition, since the wet paper side surface includes a fiber layer, very great pressure or friction is applied to the fiber by the pressing portion and thus the falling-out or cutting-out of fiber (that is, loss of fiber) from the surface of the fiber layer remarkably occurs. The loss of fiber from the surface of the fiber layer coming into direct contact with the wet paper in the paper transporting felt deteriorates the quality of paper products such as print mediums. On the other hand, since the surface of the fiber layer is roughened due to the loss of fiber, the surface smoothness of the wet paper is remarkably lowered.